The present invention relates to a coated film, more particularly, it relates to a stretched polyester film having a novel coating layer with excellent transparency, gross, anti-block properties and adhesion to overcoating.
Since stretched polyester films have excellent properties such as mechanical strength, dimensional stability, flatness, smoothness, heat resistance, chemical resistance and transparency, they have been widely used as a base film of magnetic recording medium, a film for printing, magnetic card, synthetic paper, etc.
While the polyester films have such excellent properties, they are poor in adhesion, which is a problem common to the plastic films in general. For instance, the polyester films have poor adhesion to printing inks (printing ink for cellophane, chlorinated PP ink, ultraviolet curing ink, magnetic ink, etc.), thermal transfer ink, magnetic coatings, adhesives (laminating adhesives, wood adhesives, etc.), overcoating materials (releaser, ink image receiving layer, gelatin, polyvinyl alcohol, polyvinyl acetal, cellulose acetate, cellulose butyroacetate, methyl cellulose, carboxymethyl cellulose, etc.), and deposited metals and inorganic materials (aluminum, silver, gold, ITO, silicon oxide, aluminum oxide, etc.). In recent years, attention has been focused especially on aqueous overcoatings (including ink) for lessening environmental contamination.
As a method of solving the above problem, it is known to provide a coating layer on the polyester film surface. Especially the method in which coating is conducted in the course of the film forming process is preferable in view of economic and characteristics thereof. This technique is called “in-line coating.” In a typical instance of this method, coating is conducted after longitudinal stretching and before transverse stretching, and then transverse stretching and heat setting are conducted. Such coating treatment improves adhesiveness of the film, but on the other hand, there arises a problem of tendency of the films to stick to each other (so-called blocking). In many cases, therefore, a crosslinking agent is further added to prevent blocking while improving adhesive quality of the films.
Recently, in order to reduce load on the environment, it has become an essential requirement that polyester films have good adhesiveness to water-based coating materials such as polyvinyl alcohol, polyvinyl acetal, polyvinyl pyrrolidone, methyl cellulose, carboxymethyl cellulose and gelatin. For instance, in case where a photosensitive layer utilizing a silver salt and gelatin is provided, this layer is exposed to many chemicals in the developing and fixing steps, so that such a layer is required to have enough durability to stand long-time preservation as well as strong adhesion to the base polyester film. Conventionally, for this purpose, polyvinylidene chloride, polyvinyl chloride and their copolymers have been used as undercoating. For providing even stronger adhesion, an additional undercoat containing gelatin is provided on the said layer. However, the coated films containing chlorine have a possibility of generating harmful dioxin compounds when burned. Also, the films having a silver salt coating may be withdrawn from the household or corporations to recover and reuse silver contained therein. But in this case, too, chlorine-based polymers remain in the polyester films left after recovery of silver, so that even if silver in the polymer films is recovered for reuse without burning the films, hydrogen chloride is generated when films are melted by heating. So, the development of chlorine-free undercoated polyester films has been desired.
Incorporation of a crosslinking agent in the coating composition is considered an effective way for reducing stickiness or blocking tendency, but generally addition of a crosslinking agent induces hardening of the coat. So, there arises a problem of a stretching conformability of the coating layer especially when the film is stretched after coating such as mentioned above. More specifically, if the coating layer is poor in stretchability, it fails to be stretched uniformly and may be fractured finely in the polyester film stretching step, thus forming microcracks in the coating layer in many cases. Such cracks in the coating layer cause such defects as reduction of adhesion to the overcoats, etc., and clouding of the coating layer due to scattering of light by fine unevenness of the coating layer surface. Thus, the problem arises that even when the base polyester film is not transparent, glossiness of the film surface lowers. Even if a crosslinking agent is added, there are occasions where the film-to-film sticking tendency (blocking) is not lessened as desired or, quite strangely, blocking is rather promoted. Therefore, when polyester films are coated and stretched, it is required to incorporate a crosslinking agent which meets especially the above-mentioned contradictory requirements, or a combination of such a crosslinking agent and a binder.